This invention relates generally to spacecraft, and more particularly, to techniques for effecting a transistion from one planetary orbit to another. A recurring requirement in controlling spacecraft is to be able to change the orbit of the craft, for example from a relatively low parking orbit about the earth to a much higher geosynchronous orbit, in which the period of revolution is twentyfour hours. Geosynchronous orbits are frequently used for communications satellites, weather observation, and so forth.
A space vehicle can be moved from a low circular orbit to a higher circular orbit by as few as two "burns" of a rocket motor. As is well understood, the orbit of an unpowered spacecraft, or any object, about a larger body is elliptical, with the larger body being located at one focus of the elipse. When rotation about the earth is being considered, the point in the orbit that is closest to Earth is called the perigee, and the furthest point from Earth is called the apogee. A circular orbit is, of course, merely a special case of the elliptical one. A first burn can be used to raise the apogee or high point of the orbit to a desired level, and a second burn performed at the apogee can be used to circularize the orbit. A more common technique is to employ a low-thrust motor and multiple burns. A first set of burns is used to raise the high point of the orbit, each burn occurring at the perigee or low point of the orbit. Another set of burns at the apogee is used to circularize the orbit. This approach requires a motor of much lower thrust than if only two burns are used, and the cost of the spacecraft is lower. Another important advantage is that the acceleration forces on the vehicle are low enough to permit the safe deployment of antennas and other equipment while in the low-altitude parking orbit.
In both approaches described, the position of the spacecraft has to be predicted to a high degree of precision, so that the burns can be initiated as near as possible to the low or high points of the orbit. Tracking of spacecraft for this purpose has traditionally been a function performed on the ground, based on information derived from radio signals processed at a number of tracking stations on the ground. Not only is this a complex and expensive task, but accurate prediction is especially difficult at low orbital altitudes.
For some types of missions, it would be preferable to provide the spacecraft with a form of control that was independent of the use of ground stations for position determination. In the past, building a more autonomous spacecraft has been synonymous with providing complex on-board controllers using programmable computers. Even with the high degree of autonomy provided by on-board computers, position determination must still be provided from ground stations.
Ideally, some spacecraft missions require an autonomous controller to effect transisiton from one orbit to another, but without complex on-board computers, and without continued intervention from ground stations. The present invention is directed to this end.